This application for an NIMH Mentored Scientist Developmental Award (K01) proposes a 5-year period of mentored support to expand the applicant's existing child clinical research focus to gain additional training in the methods and substance of the study of girls' aggression and antisocial behavior. The goal of the proposed research is to understand the nature, antecedents, course, and mechanisms of girls' aggression and antisocial behavior. It is the guiding contention of this proposal that the development of aggression and antisocial behavior by girls is ruled by concerns about interpersonal relationships. Specific aims are: (1) to identify unique and common developmental trajectories of aggression and antisocial behavior in early-starting boys and girls; (2) to characterize early and late starting pathways of antisocial behavior in boys and girls in two large-scale normative samples; (3) to examine gender as a moderator of risk factor-to-antisocial outcome associations; (4) to examine gender as a moderator of mediators of early externalizing behavior-to-antisocial outcome associations; (5) to conduct a qualitative analysis of cognitive, relationship, and contextual aspects of aggression; and, (6) to propose a long-term program of research. The research plan includes 2 studies: (1) secondary analyses on a large group of 151 high-risk early starter girls using data from the Fast Track (FT) project and on 2 large normative groups, the first of 197 boys and 190 girls from the normative FT study and the second of 304 boys and 281 girls from the Child Development Project (CDP). These secondary analyses will identify developmental pathways and processes that contribute to antisocial behavior and aggression; (2) a two-stage sequence of qualitative data collection, the first being an exploratory phase with normative samples of youth and professionals and the second being a systematic phase with court-involved youth, with a specific focus on social, cognitive, and contextual features of aggression and antisocial behavior. Through this mentored training and development program, the candidate will build the foundation for an independent research career focused on the identification of developmental trajectories and related processes of conduct problems in girls. This foundation will provide the necessary steps to the ultimate goals for additional longitudinal research and the development, implementation, and evaluation of a preventive intervention to reduce aggressive and antisocial behavior in girls.